1. Field of the Invention
This invention relates to a third-order nonlinear optical material excelling in third-order nonlinear optical properties characterized in terms of conversion of wavelength, phenomenon of optical bistability, and generation of phase-conjugated wavefront, and to a method for the production thereof.
2. Description of the Related Art
In recent years, the feasibility of utilizing a second-order nonlinear optical material for such optical devices as, for example, wavelength-converting elements has been under study. Such inorganic crystals as potassium dihydrogen phosphate and lithium niobate have been heretofore used as a second-order nonlinear optical material. Since these substances are used more often than not in the form of single crystals, they are deficient in processability and consequently are not easily fabricated into elements.
The feasibility of using a third-order nonlinear optical material for such optical devices as extremely high-speed optical switches, optical bistable elements, phase-correcting elements, and wavelength-converting elements is being studied. As concerns third-order nonlinear optical materials using fine metal particles such as of gold and silver, semiconducting nonoxide fine particles such as of CdS, and organic compounds exhibiting a third-order nonlinear optical effect, development of materials which satisfy such high qualities as, for example, (1) high third-order nonlinear susceptibility, (2) high transparency (small absorption) in the operating wavelength range, and (3) high response speed has been under way. The term "response speed" used herein refers to a time required for one operation in the repeated operations with the above optical device. Therefore, the smaller the response speed is, the larger the number of operations per one second can be performed. Since the time until the generated effect is vanished completely (or to such a extent as that the influence by the accumulation of effects can be neglected) is generally longer than that until the effect is manifested, the development for the latter material having a short relaxation time must be important.
As conventional technical achievements in this field, a glass having fine gold particles dispersed therein by the technique of fusion disclosed in Applied Physics A, Vol. 47, p.347 (1988) and a glass having fine gold particles dispersed therein by the technique of ion implantation disclosed in Material Research Society Symposium Proceedings, Vol. 283, p.903 (1993) may be cited. When the technique of fusion is adopted, it is at a disadvantage in limiting the species of metals usable therein by reason of fusibility and also limiting the solubility of a selected metal in glass. Since the third-order nonlinear optical properties of the relevant material are in direct proportion to the concentration of a metal contained in glass, the limitation of the solubility implies the limitation of the third-order nonlinear optical properties of the material. Even when the technique of ion implantation which allows an addition to the concentration of metal is adopted, it is still difficult to enable the glass to contain a metal at a ratio of not less than approximately 6.3 atomic percentage. As stated in Material Research Society Symposium Proceedings, Vol. 283, p.903 (1993), the glass having fine gold particles dispersed therein is observed to contain a slow relaxation component in third-order nonlinear optical effects. According to M. J. Bloemer et al., J. Opt. Soc. Am. B, 7, p.790 (1990), this phenomenon is interpreted as arising from the slow relaxation of lattice vibration of fine gold particles. Further, this technique does not deserve to be called a perfect process because it requires heat-treatment for enabling a metal ion dispersed in glass to be precipitated as the fine metal particle and consequently complicates the process.
As mentioned above, in the case of glass having fine gold particles dispersed therein as a conductive material, it is difficult to increase the concentration of the metal and consequently the heat-treatment at an elevated temperature is needed. As stated in Applied Physics Letter, Vol. 64, No. 25, p.20 (1994), there is a problem in disorder of interface because of difference in coefficient of thermal expansion between glass matrix and fine gold particle.
In J. Opt. Soc. Am., 73, p.647 (1983), it has been proposed to use a cutoff filter having fine particles of CdS.sub.x Se.sub.1-x dispersed in borosilicate glass as a third-order nonlinear optical thin film. This cutoff filter glass is manufactured by placing the raw materials for borosilicate glass and CdS.sub.x Se.sub.1-x in a platinum crucible and fusing them at a temperature of about 1000.degree. C. This manufacture, however, entails the problem that a part of semiconductor components is vaporized with variation of composition during the course of the fusion. Further, as stated in Journal of Materials Science: Materials in Electronics, 4 (1993), pp.59-61, the fine semiconducting particles such as of CdS are subjected to degradation of the third-order nonlinear optical effect and induction of the phenomenon of blackening and have dubious stability as evinced by precipitation of sulfur due to oxidative decomposition. Further, the filter glass warrants no perfect safety because it contains cadmium, a substance harmful to a human body, and the process used for the production thereof hardly deserves to be called satisfactory in terms of processability because it requires the raw materials to be fused at an elevated temperature.
On the other hand, techniques which comprise dispersing fine CdS particles in a polymer in order to avoid harmful effects generated by the heat-treatment at an elevated temperature have been disclosed in JP-A-04-189801 and JP-A-05-184913. In these publications, techniques which comprise forming CdS by the light irradiation to obtain CdS particles having its surface coated with a polymer, and forming the particles into a film have been disclosed.
These publications, however, have defects in that the stability of these fine particles is degraded with the elapse of time because of easy oxidative decomposition of CdS itself.
In the dispersions of these fine particles, the conducting particles of gold or the semiconducting particles of CdS which are dispersed in an insulating medium can be expected to manifest prominent third-order nonlinear optical properties, ultraradiation of excitons, and specificity of crystal interface reaction by the fact that electrons and positive holes, or excitons are confined three-dimensionally. In order to ensure thorough derivation of this quantum size effect, the fine particles dispersed are required to possess a size small enough to manifest the quantum size effect, i.e., a size of not more than 500 nm, preferably not more than 100 nm, exhibit a narrow particle diameter distribution, and assume the best possible state of dispersion.
Further, Nature, Vol. 374, No. 6523, pp.625-627 discloses that an oxide thin film which is formed by the thermal decomposition of alkyl carboxylate of V, Cr, Mn, Fe, Co, Cu, and Ni can manifest large third-order nonlinear optical effects.
As stated in JP-A-07-284516, a thin film formed of an oxide of a transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, and Cu and containing an additive component selected from the group consisting of Al.sub.2 O.sub.3, ZnO, and ZrO.sub.2 has been proposed. The manufacture of this thin film can hardly be called easy because it necessitates a heat treatment at an elevated temperature in spite of the addition of A1.sub.2 O.sub.3, ZnO, or ZrO.sub.2. Further, the additive component is only effective in improving the thin film instructural stability and mechanical strength and is not particularly effective in enhancing the third-order nonlinear optical property of the thin film.
Polymer, Vol. 34, No. 6 (1993), pp.1174-1178 discloses such data as the structures and the characteristics (such as third-order nonlinear susceptibility coefficient) of a poly(p-phenylenevinylene) (PPV)/silica composite and a PPV/V.sub.2 O.sub.5 composite. The section "INTRODUCTION" of the literature has a mention to the effect that when the PPV, a substance which inherently manifests a third-order nonlinear optical effect, incorporates silica or V.sub.2 O.sub.5 therein, it enjoys an improvement in the third-order nonlinear optical effect thereof. It, however, has absolutely no mention of the fact that silica or V.sub.2 O.sub.5 itself manifests a third-order nonlinear optical effect. Further, the PPV itself is deficient in manufacture of a film because it is insoluble and infusible. PPV, therefore, must be obtained by previously forming a film using a water-soluble sulfonate thereof as the precursor and thermally decomposing the film.
This invention aims to solve the problems of processability attendant on the conventional techniques and also improve the speed of relaxation. To be specific, it aims to provide a material which excels in processability and has a higher relaxation speed as compared with the conventional material by introducing an oxide of a transition metal into a transparent polymer.
An object of this invention, therefore, is to provide a novel third-order nonlinear optical material and a method for the production thereof.
A further object of this invention is to provide a third-order nonlinear optical material satisfying the requirements, i.e. (1) large third-order nonlinear susceptibility; (2) high transparency (small absorption) in the operating wavelength range; and (3) high relaxation speed, and being excellent in weatherability to withstand the deterioration with the elapse of time such as due to the decomposition through oxidation, heat resistance to withstand the thermal deterioration such as the alteration of composition, chemical and thermal stability (lightfastness) to withstand the laser beam, safety to a human body, and processability manifested during the course of manufacture.
Another object of this invention is to provide a third-order nonlinear optical material which is capable of fully manifesting a quantum size effect.
Still another object of this invention is to provide a third-order nonlinear optical material which abounds in species of substances usable (or fit for arbitrary selection) for raw material components, prevents restrictions on manufacture from affecting the properties of raw materials, and enjoys high relaxation speed.
Yet another object of this invention is to provide a method for the production of a third-order nonlinear optical material which satisfies the objects mentioned above and requires no complicate treatment or process, avoids necessity of such a heat treatment as by heating at an elevated temperature or fusion, and excels in processability.